System for providing scrambled content, and system for descrambling scrambled content

ABSTRACT

A system for providing scrambled content, comprising a control word generator, each control word including a control word identifier, a scrambler for providing a stream of scrambled data packets, and an encryption device for providing entitlement control messages (ECM&#39;s). One or more consecutive packets are scrambled using the same control word (CW). Each packet includes a control word identifier identifying the control word used. The ECM encryption device provides ECM&#39;s including a previous control word (CW P ), a current control word (CW C ) and a next control word (CW N ). A system for descrambling scrambled content comprises a descrambler for descrambling the scrambled content and a decryption device for decrypting ECM&#39;s to obtain control words. The ECM decryption device delivers control words to the descrambler and the descrambler descrambles the data packets of the scrambled content using a control word having a control word identifier corresponding with the control word identifier of the data packet to be descrambled. The system further comprises a storage device for storing scrambled content and means to control play back of the stored content, going fast forward and going backwards. The system is programmed to extract ECM&#39;s and to provide the ECM&#39;s to the ECM decryption device for decryption. The system is programmed to request the ECM decryption device to provide at least a next control word (CW N ) at play back or going fast forward, and to provide at least a previous control word (CW P ) and a current control word (CW C ) at going backwards.

CROSS-REFERENCE TO OTHER APPLICATIONS

This Application is a National Phase of International Application No.PCT/EP01/03186, filed on Mar. 21, 2001, which claims priority fromEuropean Patent Office Patent Application No. 00201277.1, filed on Apr.7, 2000.

The invention relates to conditional access systems suitable forreceiving apparatus provided with storage devices with large capacity.The invention specifically relates to a system for providing scrambledcontent according to the preamble of claim 1 and to a system fordescrambling scrambled content according to the preamble of claim 3.

Recently television receiving apparatus have been equipped with storagedevices with large capacity allowing the users to store hours of videocontent. In this manner the user can create a “personal television”recording his favourite programs and the like. Such receiving apparatuscan be made as a television apparatus, a set-top box or the like.

The content stored in the storage devices, such as large hard discs, caninclude scrambled content with conditional access, wherein control wordsare needed to descramble the content. Such storage devices allow goingbackwards through the scrambled content, wherein however the presentlyavailable mechanisms for cycling control words become difficult to use.Going backwards is necessary as the users would like to use a VCR-likerewind functionality. However the current techniques used for controlword cycling are designed for signals that go forward only.

The invention aims to provide a system for providing scrambled contentand a system for descrambling scrambled content of the above-mentionedtype, wherein a VCR-like rewind function is supported.

According to the invention a system for providing scrambled content,comprises a control word generator, each control word including acontrol word identifier, a scrambler for providing a stream of scrambleddata packets, wherein one or more consecutive packets are scrambledusing the same control word (CW) and wherein each packet includes acontrol word identifier identifying the control word used, and anencryption device for providing entitlement control messages (ECM's),each ECM including at least a next control word (CW_(N)), characterizedin that the ECM encryption device provides ECM's including a previouscontrol word (CW_(P)), a current control word (CW_(C)) and a nextcontrol word (CW_(N)).

In a second aspect of the invention a system for descrambling scrambledcontent is provided, comprising a descrambler for descrambling thescrambled content, a decryption device for decrypting ECM's to obtaincontrol words, wherein the ECM decryption device delivers control wordsto the descrambler, wherein the descrambler descrambles the data packetsof the scrambled content using a control word having a control wordidentifier corresponding with the control word identifier of the datapacket to be descrambled, the system further comprising a storage devicefor storing scrambled content and a processing unit with means tocontrol playback of the stored content, going fast forward and goingbackwards, wherein the processing unit is programmed to extract ECM'sand to provide the ECM's to the ECM decryption device for decryption,characterized in that the processing unit is programmed to request theECM decryption device to provide at least a next control word (CW_(N))at play back or going fast forward, and to provide at least a previouscontrol word (CW_(P)) and a current control word (CW_(C)) at goingbackwards.

By providing entitlement control messages with three control words, i.e.the current, next and previous control words, the receiving apparatuscan playback stored content from disc in a normal manner, whereinfurther fast forward and rewind functions are available. When thereceiving apparatus is going backwards through the content, theprocessing unit picks up the first ECM it finds, sends the ECM to theECM decryption device and requests the decryption device to delivercurrent and previous keys and loads these keys into the descrambler.Processing the ECM's and synchronizing the provision of control words isrelatively simple in this manner.

The invention will be further explained by reference to the drawings inwhich an embodiment of the systems of the invention is shown.

FIGS. 1A and 1B show simplified diagrams of embodiments of the systemsfor providing scrambled content, and descrambling scrambled contentaccording to the invention.

FIGS. 2A and 2B show diagrams to explain the operation of the systems ofthe invention.

FIG. 1 shows a system 1 for providing scrambled content comprising acontrol word generator 2 and a scrambler 3. The scrambler 3 receivesclear content and delivers content scrambled using the control wordsprovided by the control word generator 2 as encryption keys. The controlword generator provides a new control word every ten seconds forexample. As is well-known in the art, the control words are generallyused as a seed for a pseudo-random binary sequence generator, whereinthe output of the PRBS generator is used for scrambling the clearcontent. Of course other scramble systems such as a block cypher systemmay also be used. As such scrambling systems are known per se, this isnot discussed in detail in this specification as it is not part of thepresent invention. Each control word CW has an associated control wordidentifier, which in case of the generally used MPEG system is only onebit, i.e. either a zero or a one. In a corresponding manner the samecontrol word identifier, i.e. a zero or a one, is associated with eachscrambled data packet or plurality of scrambled data packets scrambledunder the control of the control word having the same associatedidentifier zero or one.

The control words CW₀ and CW₁ are also delivered to an ECM encryptiondevice 4 which encrypts the control words using an input key P. Theencryption device 4 can be made as a smart card. The encrypted ECM'swith the control words are inserted into the scrambled content andbroadcasted or delivered in any other manner to a number of subscriberseach having a system 5 for descrambling scrambled content.

It is noted that the key P used by the encryption device 4 can betransferred to the systems 5 in so-called entitlement managementmessages which are not shown in FIG. 1. Conditional access systemsoperating with such a hierarchy of keys are known per se and are notfurther described in this specification.

The descrambling system 5 comprises a descrambler 6 for descrambling thescrambled content and a decryption device 7 for decrypting ECM's toobtain the control words CW. This device 7 can be made as a smart card.Further, the system 5 comprises a processing unit 8 controlling theoperation of the system and having a schematically indicated controlmeans 9 allowing control of the system by the user. The scrambledcontent is received by the processing unit 8 and can be stored on astorage device 10, for example a hard disc with large capacity. Theprocessing unit 8 forwards the scrambled content to the descrambler 6and extracts the ECM's from the stream and forwards the ECM's to thedecryption device 7. The decryption device 7, generally made as a securedevice such as a smart card, decrypts the ECM's received and ascontrolled by the processing unit 8 delivers the control words CW₀, CW₁to the descrambler 6. If a data packet with the control word identifier0 is received the control word CW₀ is used, if a data packet having thecontrol word identifier 1 is received, the control word CW₁ is used.

In the known conditional access system either the current control wordCW_(C) and the next control word CW_(N) is included in the ECM's or onlythe next control word CW_(N). The decryption device 7 decrypts thecontrol words and loads the control words into the descrambler 6. Asstated above, the data packets passing the descrambler 6 havecorresponding control word identifiers indicating which control word CW₁or CW₀ to use. In normal use, when only going forward, an ECM isextracted from the stream and will at least contain the control word CW₁or CW₀ to be used at the next transition from identifier 1 to 0 in thedata packet stream. However when going backwards through the datastream, the ECM at any location will not have the control word CW_(P)for the previous data packet. This means that the processing unit 8 mustlook further back then the current data packet being processed by thedescrambler in order to find a previous ECM. This would require anintensive operation and would mean a high load on the processingcapacity of the system 5.

According to the invention, this problem of locating an ECM in the datastream is avoided by including in the ECM's three keys, i.e. theprevious control word CW_(P), the current control word CW_(C) and thenext control word CW_(N). In this manner each ECM extracted from thestream by the processing unit 8 at play back of the content stored onthe disc 10 contains three control words, including the control wordrequired for descrambling the previous data packet. In this manner arewind function is supported in an easy manner so that the user canscroll back through the content retrieved from the storage device 10using the control means 9. Of course, such control means may include aremote control device.

An illustration of the operation of the systems of the invention isschematically shown in FIG. 2. A content stream with data packets A, B,C and D is shown, wherein it is assumed that data packet A has thecontrol word identifier 0, the data packet B the identifier 1, the datapacket C the identifier 0 etc. The ECM stream is shown above the datapacket stream. As indicated, a new ECM is extracted from the streamshortly before a transition from data packet A to B, from B to C etc.The ECM extracted from the data stream shortly before the beginning ofdata packet A, includes the previous control word CW_(P1), the currentcontrol word CW_(A0), and the next control word CW_(B1). The next ECMincludes the previous control word CW_(A0), the current control wordCW_(B1) and the next control word CW_(C0). During normal playback, theprocessing unit 8 will extract the ECM's subsequently from the datastream and will send the ECM's to the decryption device 7. Theprocessing unit 8 will request the decryption device 7 to send thecurrent control word CW_(A0) and the next control word CW_(B1) to thedescrambler 6. The descrambler 6 receiving the data packet A withidentifier 0, will use the control word CW_(A0) to descramble this datapacket. At the transition from data packet A to data packet B, the newidentifier 1 indicates the descrambler 6 to use the control wordCW_(B1). The same operation applies to the fast forward mode wherein theprocessing unit 8 will request the decryption device 7 to send thecurrent and next control words to the descrambler 6.

If the user operates the control means 9 for going backwards, i.e. aVCR-like rewind function, the processing unit 8 instructs the decryptiondevice 7 to provide the current control word CW_(A0) and previouscontrol word CW_(P1). In this manner the descrambler 6 can descramblethe previous data packet.

In the above example it is indicated that the ECM's are stored as partof the content. It is also possible to store the ECM's separately withtiming information. In this case timing information in the contentstream is used by the processing unit 8 to extract or retrieve thecorrect ECM's from the storage device 10.

From the above it will be understood that the invention providessystems, wherein going backwards through the stored video content isallowed without any significant increase in the load on the processingunit in processing ECM's and synchronizing control words. It will beclear that the invention can be used with any type of video or audiocontent.

The invention is not restricted to the above described embodiments whichcan be varied in a number of ways within the scope of the attachedclaims.

1. System for descrambling scrambled content, comprising a descrambler for descrambling the scrambled content, a decryption device for decrypting ECM's to obtain control words, wherein the ECM decryption device delivers control words to the descrambler, wherein the descrambler descrambles the data packets of the scrambled content using a control word having a control word identifier corresponding with the control word identifier of the data packet to be descrambled, the system further comprising a storage device for storing scrambled content and a processing unit with means to control play back of the stored content, going fast forward and going backwards, wherein the processing unit is programmed to extract ECM's and to provide the ECM's to the ECM decryption device for decryption, characterized in that the processing unit is programmed to request the ECM decryption device to provide at least a next control word (CW_(N)) at play back or going fast forward, and to provide at least a previous control word (CW_(P)) and a current control word (CW_(C)) at going backwards.
 2. System according to claim 1, wherein the processing unit is programmed to request the ECM decryption device to provide the current control word (CW_(C)) together with a next control word (CW_(N)) at playback or going fast forward and to provide the current control word (CW_(C)) together with a previous control word (CW_(P)) at going backwards.
 3. System according to claim 1, wherein the ECM decryption device is accommodated in a smart card. 